T1 channel bank control process and apparatus

ABSTRACT

A technique for powering telephone lines using an unbalance current source and current sink; and a technique for improving attenuation/frequency distribution and return loss (impedance matching) of transformer-coupled wire-line communications circuits by using secondary series capacitance and an AC current pump signal source; and a generation of ringing voltage as positive voltage pulses with respect to a negative power supply voltage; and a technique for removal of AC power ripple by using an active linear floating filter for the purpose of powering telephone line circuits, and a technique for injection of real time tone samples into T1 transmissions circuits by use of a T1 framer idle code register. The combination of the above five circuit techniques provides for the hardware implementation of a single printed circuit board embodiment (Line Interface Unit LIU) of a plurality of communications functions including a T1 channel service unit, a ringing generator, power converters, a ringback tone generator, and a channel bank controller. The LIU supports selectable T1 standards of communications. Dual 12-channel telephone line voice cards plug into the LIU card to provide a complete T1 channel bank control process and apparatus. The invention solves telephone line interface, power filtering, ringing generation, and tone injection problems with lower component complexity, costs, and physical size than prior art solutions.

This application is a division of application Ser. No. 08/440,099, filedMay 12, 1995, currently pending.

FIELD OF INVENTION

The present invention relates to a single printed circuit boardembodiment(Line Interface Unit LIU) of a plurality of communicationsfunctions including a T1 channel service unit, a ringing generator,power converters, a ringback tone generator, and a channel bankcontroller. The LIU supports selectable T1 standards of communications.Dual 12-channel telephone line voice cards plug into the LIU card toprovide a complete T1 channel bank control process and apparatus.

BACKGROUND OF THE INVENTION

The two basic types of business systems are in common use. The KeyTelephone System (KTS) serves small businesses where a few people needaccess to any one of several communication paths to a switching system.By using push buttons with indicator lights on the telephone instrument,the user can select an idle line to make a call. The user may alsoidentify an incoming call on any of the lines and connect to that lineto answer the call by operating the appropriate button. In FIG. 1, theABC warehouse (8020) uses a key telephone system (8001). It is connectedto a switching machine (not shown) in an end central office (8010)through a digital T1 line (8021). An Access Bank™ (the trademark for thepresent invention) channel bank (8002) converts the T1 line to 12 or 24loop-start dialog telephone lines (8022) that connect the Key TelephoneSystem (8001). Since the warehouse telephone stations are operated byhuman beings, the communications path represents a human-to-machineinterface and is called a KTS Line.

The second category of business system is known as a PBX or PrivateBranch Exchange. This is a switching machine, similar to thoseconnecting subscribers or trunks in central offices. PBX's located onthe subscriber premises are considered "branches" or subsidiaries of thecentral office switching system. They are "private" because they arededicated to the business subscriber for the use of in-house personnel,instead of being shared with many business and residential users, likethe switches in telco central offices. As indicated in FIG. 1, the ABCsales office (8023) and factory (8024) use PBX's (8011, 8008) for theirbusiness telephone systems. The communication path between the PBX's(8011, 8008) and their CO's (8004, 8010) are machine-to-machineinterfaces, thus they are PBX trunks (8025, 8026) and require trunkcircuits.

PBX trunks may be connected as many individual analog lines (8026) froma central office (8010). Alternatively, PBX trunks may be provided on adigital T1 line (8027) from the Central office (8004). An Access Bank™channel bank (8005) is used to convert the T1 line to 12 or 24ground-start telephone lines (8025) within, or near, the ABC SalesOffice (8023). PBX stations are connected to the PBX switching machine,just like residential subscribers connect to the end CO switch, and areknown as station lines. PBX station users can dial one another and beconnected by the PBX switch. If a PBX user wishes to call a telephonelocated outside the company, the PBX switch selects an idle trunk overwhich the call is dialed, or in some cases a PBX attendant places thecall using a special control box or turret (not shown), just liketelephone operators used to do from their switch boards. Similarly,incoming calls over the CO trunks are received by the PBX switchingsystem and extend to the desired station automatically or by anattendant.

Note that, unlike key telephone system users, the PBX station cannotselect a particular trunk to answer or to initiate an outgoing call. ThePBX switching system does that. However, as can be seen in the ABCFactory (8024) in addition to individual station lines, the PBX alsoserves a Key Telephone System (KTS)(8028). This provides the userswithin a department the convenience and features of the KTS (8028) inanswering and making calls through the PBX switch. This arrangement isreferred to as a key system installed "behind" a PBX. The communicationpath between PBX and KTS is a human-to-machine interface and is called akey system line.

Three types of special arrangements shown in FIG. 1 are popular inbusiness communications. PBX users might want trunks to a central officewhich are not from the CO serving them. Such a transmission facility isknown as a Foreign Exchange (FX) Trunk (8029). Companies might find itconvenient to directly interconnect their PBX's at different locationswithout switching over shared public trunks. These arrangement arecalled tie trunks, and provide full period dedicated private circuitsfor interconnection. The ABC Sales Office (8023) and ABC Factory (8024)are connected by a digital T1 line (8036) to transmit the trunkcircuits. Access Banks™ at the sales office (8034) and Factory (8035)serve to convert the T1 line (8036) to 12 or 24 analog tie trunk circuitpaths (8033).

There might be a need for PBX stations to be located at distant places,far removed from the premises where the business system serves themajority of the user personnel. Such arrangements can be engineered andtreated just like any other station serving the PBX. This is known as anOff-Premises Extension (OPX). A T1 line (8007) is used to carryoff-premise extension channels from the Digital PBX (8008) to theWarehouse (8030). An Access Bank™ (8009) in the Warehouse converts theT1 line (8007) to 24 individual telephone connections (8031).

For providing residential telephone service, a T1 line (8012) is used tocarry 24 telephone channels to a common point of distribution (8014).Only 4 wires (2 pairs) are used to connect this point to the Centraloffice (8013) with a digital T1 line (8012), rather than 48 wires (24pairs) required when using analog transmission. An Access Bank™ (8015)is used to convert the T1 line to 24 residential telephone lines (8037)at the point of distribution (8014).

PRIOR ART DISCUSSION

The primary prior art of powering a line or trunk circuit is shown inFIG. 10. -48 volt battery is fed through relay A to one winding oftransformer T1 and to the Ring lead of the line or trunk. Ground isconnected to relay A similarly and to the second winding of T1 and thento the Tip lead of the line or trunk. When there is a closed circuitbetween the Tip and Ring current flows through the windings of relay Ain a series aiding manner, operating relay A and indicating that thecircuit is in use. Current through the winding of T1 provides power tothe telephone or trunk. Because several milliaperes is required to powera telephone set or indicate an operating trunk, the transformer T1 mustbe capable of carrying this current without saturating its magneticcore. The resultant size of transformers which will avoid saturation andhave the necessary high inductance for good transmission quality arerelatively large, heavy and costly. Current typical transformers forthis purpose are about 1 cubic inch in size and weigh several ounces.

Technique for Improving Attentuation/Frequency Distortion and ReturnLoss (Impedance Matching) of Transformer-Coupled Wire-Line CommuncationsCircuits by Using Secondary Series Capacitance and an AC Current PumpSignal Source.

The coupling of voice band and digital transmission systems to cablepairs exposed to lightning, static electric discharges and accidentalconnection to power sources typically utilize transformers. By using atransformer line coupling method, only the power supply of thecommunication system is exposed to the above hazards. These powersupplies are ruggedly designed to self-protect and are also securelygrounded.

The second requirement in coupling to a single cable pair in a fullduplex voice system is to adequately separate the transmitted signalfrom the received signal. Failure to achieve at least 15 dB ofseparation results in objectionable or intolerable echoes to both endsof the system. Transformers and/or amplifying arrangements can achievethe needed separation. Such systems are known as transformer orelectronic hybrid arrangements.

When space, weight and cost are not the primary requirements, arelatively large 4 or 6 winding transformer and suitable balancingnetwork may be used. To prevent excessive loss of the voice band signal,the hybrid windings must be tightly coupled and to prevent saturation ofthe core by the DC line current a large core cross-section is required.Separation of incoming and outgoing signals of 30 decibels (signal powerratio) is typically achieved. FIG. 11 shows the transformer hybridarrangement.

A second alternative is the use of a 2 winding transformer. The linecurrent may be supplied as in FIG. 12, with amplifying arrangements canseparate the incoming and ongoing signals.

The electronic hybrid arrangement of FIG. 12 requires that the BalanceNetwork terminate the transmission line to prevent an echo back to thecable pair and to ensure that the voltage from the line driver amplifiercan be effectively canceled in the subtracting circuit. The output ofthe subtracting circuit is the incoming signal with a minor residue ofthe outgoing signal.

The current competitive market for telephone equipment has placed a highpriority on equipment size, weight and cost. The reduction of size andcost of the coupling transformer has become the focus of some systems.FIG. 13 shows the use of negative feedback from the line side of thetransformer to improve the frequency response and control the outputimpedance of the driving amplifier. This arrangement is utilized intraditional high fidelity, sound systems. In this case, the miniaturizedtransformer utilizes a low inductance core which will not saturate whencarrying the line current. The series resister R₁ and the real andcomplex values of the feedback network also adjusts the phase andmagnitude of the output of the Receive (REC) amplifier. The cancellationof the outgoing signal (receive) from the incoming signal (transmit) isaccomplished in the summing circuit (Σ) at the input of the Xmtamplifier.

The above descriptions are examples of prior art. Injection of Real TimeTone Samples Into T1 Transmission Circuits By Use Of A T1 Framer IdleCode Register

The use of digitized call-progress tones, supervision tones and recordedannouncements into PCM bit streams extends from T1 carrier systems topersonal computers. Typically, these systems use an adjunct device tomultiplex the digital bit stream into the main data stream. In this newinvention, an economy of components is achieved by utilizing the IdleCode Register of the T1 framer or T1 controller to insert a new tonesample in a dynamic fashion.

Generation of Ringing Voltage As Positive Voltage Pulses With Respect ToA Negative Power Supply Voltage

Prior art of ringing generators ranges from hand-cranked magnetics builtinto multipart, line telephones and small PBXs, DC motor-driven 20 Hzgenerators, sine wave oscillators coupled to high-power, high-voltageamplifiers, magnetic sub-harmonic generators driven by 60 Hz power andother arrangements of switches and capacitors. However the need forsmaller, more efficient and less expensive generation of 20 Hz, 85VRMSringing sources persists.

Removal of AC Power Ripple By Using An Active Linear Floating Filter forthe Purpose of Powering Telephone Line Circuits.

Conventional fixed voltage-regulated linear Subscriber Line InterfaceCircuits SLIC power sources dissipate a significant amount of power andrequire a means of dissipating the resultant heat. This is necessary toaccommodate the typical range of ±20% voltage fluctuations fromcommercial power mains.

A second common solution is to rectify the 60 Hz power source, 50 Hz oruse a switched power supply, employing pulse-width modulation (PWM) toaccomplish voltage regulation. Switched power supplies operate at highfrequency and utilize high-frequency transformers to achieve the desiredvoltage levels and isolation from the power line and then use any ofmany rectification methods, to develop the DC voltage which is fed backto the control circuit for regulation of the PWM circuitry. This systemprevents the heat generation of the fixed voltage-regulated power systembut generates high-frequency signals which must be eliminated byfiltering and shielding.

The new power filter circuit described here maintains a constant DCvoltage drop and an AC voltage drop nearly equal and opposite to theripple of the rectifier output.

The present invention solves telephone line interface, power filtering,ringing generation, and tone injection problems with lower componentcomplexity, costs, and physical size than prior art solutions listedabove.

GLOSSARY FROM ENTRIES

24th-Channel Signaling: Digital signal level-1 (DS1) signaling for whichthe signaling for each of the first 23 channels is multiplexed onto the24th channel, thereby providing a full 64 kbps for user data on each ofthe first 23 channel. Also called clear-channel signaling. See alsocommon-channel signaling and primary rate interface.

A/D Analog-to-Digital conversion: The process of encoding analog signalsinto digital signals.

AB bits Common label for in-band signaling bits on DS1 SF or 2-Mbit/s31-channel (non-CAS) signals. See also signaling bits.

ACO Alarm Cut-Off: A switch or state that disables alarm outputs.

AIS Alarm Indication Signal: The AIS indicates a digital facilityfailure. Originally called a "Blue Signal". The AIS is an "all-ones"pattern.

AMI Alternate Mark Inversion: A line code in which a binary zero isrepresented by zero-voltage interval, and binary ones are represented byalternating positive and negative voltage pulses.

Analog Voice Terminals: An Analog voice terminal (telephone) receivesacoustic voice signals and sends analog electrical signals along theline. These voice terminals are served by a signal wire pair (tip andring). The Model 2500 telephone set is a typical example of an analogvoice terminal.

Analog: The representation of information by means of continuouslyvarying physical quantities such as amplitude, frequency, phase, orresistance.

Automatic Number Identification (ANI) T1.104-1988!: ANI provides thebilling number of the line or trunk that originated a call.

B8ZS (bipolar with 8-zero substitution) T1.401-1989!: A code in whicheight consecutive "zeros" are replaced with the sequence 000+ -0- + ifthe preceding pulse was +, and with the sequence 000- +0+ - if thepreceding pulse was -, where + represents a positive pulse, - representsa negative pulse, and 0 represents no pulse.

Bandwidth: The difference, expressed in hertz, between the highest andlowest frequencies in a range of frequencies that determine channelcapacity.

Bipolar (alternate mark inversion) signal T1.403-1989!: A pseudoternarysignal, conveying binary digits, in which successive "ones" (marks,pulses) are of alternating, positive (+) and negative (-) polarity,equal in amplitude, and in which a "zero" (space, no pulse) is of zeroamplitude.

Bipolar Signal: A digital signal that uses either a positive or negativeexcursion, usually alternating, for one state and ground for the other.

Bipolar Violation (BPV) The occurrence of a pulse that breaks thealternating pulse polarity rule. BPV measurements do not include pulsesviolated by zero substitution codes.

Bipolar Violation T1.403-1989!: In bipolar signal, a one (mark, pulse)which has the same polarity as its predecessor.

Bit (binary digit): One unit of information in binary notation, havingtwo possible states or values: 0 or 1.

Bit Period (T) T1.106-1988!: The amount of time required to transmit alogical one or a logical zero.

Bit Rate: The speed at which bits are transmitted, usually expressed inbits per second. Also called data rate. See also baud and bits persecond.

Bits Per Second (bps): The number of binary units of information thatare transmitted or received per second. See also baud and bit rate.

BPS: See bits per second.

Byte: A sequence of (usually eight) bits processed together.

C-Message (CMSG) In analog measurements, a specific filter that measuressignal noise in a standard telephone subscriber environment.

Central Office (CO) The location of telephone switching equipment thatprovides local telephone service and access to toll facilities forlong-distance calling. More than one CO can serve the same area.

Central Office (CO) Trunk: A telecommunications channel that providesaccess from a PBX to the public network through the local centraloffice.

Channel Bank: Terminal equipment for a transmission system used tomultiplex individual channels using frequency-division multiplexing(FDM) or time-division multiplexing (TDM).

Channel: A telecommunications transmission path for voice and/or data.

Circuit: 1. An arrangement of electrical elements through which electriccurrent flows, providing one or more specific function. 2. A channel ortransmission path between two or more points.

CO: See central office.

Communications System: The software-controlled processor complex thatinterprets dialing pulses, tones, and/or keyboard characters and makesthe proper interconnections both within the system and external to thesystem. The communications system itself consists of a digital computer,software, storage device, and carriers with special hardware to performthe actual connections. A communications system provides voice and/ordata communication services, including access to public and privatenetworks, for telephones and data terminals on a customer's premises.See also switch.

CPE: See class of service.

CSU Channel Service Unit: Equipment at customer premises that terminatesdigital circuits, and provides testing functions.

CSU: See central processing unit.

Customer Provided (premises) equipment (CPE): Customer owned equipmentthat is not provided as part of the system but is to be connected to it.

Customer Service Unit (CSU): See network channel-terminating equipment.

Cyclic Redundancy Check (CRC) T1.403-1989!: A method of checking theintegrity of received data, where the check uses a polynomial algorithmbased on the content of the data.

D/A Digital-to-Analog conversion: the process of decoding analog signalsfrom digital signals.

D4 The common name for DS1 SuperFrame format. See also Super-FrameFormat.

D4 Framing Format: A format containing 12 frames. See also extendedframe and frame.

DACS Digital Access and Cross-connected System. See also DigitalCross-connect System.

Data Rate: See bit rate.

dBDSX, dBdsx Decibels relative to the nominal DSX signal level. Seedecibel.

dBm Decibels relative to one milliwatt (1 mW). See also decibel.

Dedicated Line: Also known as a private or leased line. It is for theexclusive use of the leasing party.

Demultiplexer: A device used to separate two are more signals that werepreviously combined by a compatible multiplexer and transmitted over asingle channel.

Digital Circuit T1.206-1988!: A combination of two digital transmissionchannels permitting bi-directional digital transmission in bothdirections between two points, to support a single communication.

Digital Data: Data represented in discrete, discontinuous form, usuallybinary. This is in contrast to continuous analog data, usuallyrepresented in sine wave form.

Digital Exchange T1.206-1988!: An exchange that switched digital signalsby means of digital switching.

Digital Loopback T1.206-1988!: A mechanism incorporated into a terminalor into the network whereby a duplex communication path my be connectedback upon itself so that the digits sent on the transmit path arereturned on the receive path.

Digital Path T1.206-1988!: The whole of the means of transmitting andreceiving a digital signal of specified rate between the two digitaldistribution frames (or equivalent) at which terminal equipments ordigital exchanges will be connected.

Digital Signal, Level n (Dsn) T1.101-1987!: Digital signal level in thetransmission hierarchy. Bit rates at each level are as follows:

    ______________________________________                                        Level               Bit Rate                                                  ______________________________________                                        0                   64 kb/s                                                   1                   1.544 Mb/s                                                2                   6.312 Mb/s                                                3                   44.736 Mb/s                                               ______________________________________                                    

Digital Signal Cross Connect, Level n (DSX-n) T1.101-1987!: Convenientcentral point for cross-connecting, rearranging, patching and testingdigital and testing digital equipment and facilities at the Dsn level.

Digital Transmission: A mode of transmission in which the information tobe transmitted is first converted to digital form and then transmittedas a serial stream of pulses.

Digital: The representation of information in discrete elements such asoff and on or 0 or 1.

Direct Distance Dialing (DDD): The capability of completinglong-distance calls without operator assistance. Direct Inward Dialing Afeature that allows an incoming call from the public network (not FX orWATS) to reach a specific telephone without attendant assistance. DIDcalls to DID-restricted telephone lines are routed to an attendant orrecorded announcement, depending on the option selected.

Disconnect Signal T1.104-1988!: An on-hook signal indicating theconnection is being cleared. It is initiated by the disconnect-controloffice (except under maintenance conditions) and is repeated through thetrunks composing an established connection. The signal responding to adisconnect signal, but applied in the direction opposite to thedirection of propagation of the disconnect signal, may also beconsidered a disconnect signal.

DS1 Digital Signal level 1: 1.544 Mbit/s; also called "T1" (24 DSOchannels).

DS1 (digital signal level 1) T1.403-1989!: A digital signal transmittedat the nominal rate of 1.544 Mbit/s.

DS1: See digital signal level 1.

DS1: Robbed-Bit Signaling See robbed-bit signaling.

DSO Digital Signal level 0: typically 64 kbit/s to 56 kbit/s. DSO mayalso refer to an individual channel in a DS1 or higher rate signal.

DSO×N DSO times N: refers to a signal composed of N number of DSOs. Thesignal's rate is determined by multiplying the number of DSOs by therate of a single DSO. Example: a DSO×4 signal, where one DSO is 64kbit/s would form a 64×4, or 256 kbit/s signal. See also DSO and N×64.

E&M E-Lead and M-Lead: two of the leads in a six wire telephone circuit.Also refers to a supervisory signaling scheme with register digits andcodes conveyed on these leads.

Exchange Carrier (EC) T1.104-1988!: A carrier authorized to provedtelecommunication services within one or more access service areas.

Exchange Carrier (EC) T1.502-1988!: The telecommunications commoncarrier franchised to provide telecommunications services within one ormore exchanges. An EX may also provide exchange access service,intra-LATA long-distance service, and in some unusual cases, interLATAservice.

Extended Superframe (Fe) Framing Format: A format of 24 frames. See alsoframe.

Facility: The equipment constituting a telecommunications transmissionpath. See also line and trunk.

Foreign Exchange (FX): See FX.

Format Structure T1-201-1987! T1.205-1988!: A combination of two or moredata elements grouped in a prescribed sequence.

Fractional T1 (FT1) Referring to sub-rate signals composed of severalchannels within a T1 (DS1) signal. The rate of an FT1 signal isdetermined by multiplying the number of selected channels (DSOs) by theDSO base rate.

Frame: One of several segments of an analog or digital signal that has arepetitive characteristic. For example, in a time division multiplexed(TDM) system, a frame is a sequence of time slots, each containing asample from one of the channels served by the multiplex system. Theframe is repeated at the sampling rate, and each channel occupies thesame sequence position in successive frames. See also D$ framing formatand extended superframe framing format.

Full-Duplex Transmission: A transmission system capable of carryingsignals in both directions simultaneously.

FX (foreign exchange): A central office (CO) other than the oneproviding local access to the public network.

FX Trunk: A telecommunications facility that connects a communicationssystem to a central office (CO) other than its own.

Glare: The simultaneous seizure of a two-way trunk by two communicationssystems, resulting in a standoff.

Hand-Up Signal T1.104-1988!: An on-hook signal sent from an end officetoward the disconnect-control office indicating either calling or calleduser hang-up and requesting the connection be disconnected. Theinterface remains dedicated to the call until the disconnect-controloffice responds to the hand-up signal. Hang-Up T1.104.1988! Calling orcalled user placement of a telephone set or other unit oftelecommunications equipment in the quiescent state.

Hz Hertz: A frequency measurement that indicates the number of cycleswhich pass a specific point per second. One Hertz equals one cycle persecond.

impedance: The effect of resistance, inductance, and capacitance on atransmitted signal. Expressed in ohms (Ω).

In-Band T1.403-1989!: Using or involving the information digit timeslots of a DS1 frame; i.e., bit assignments of a frame exclusive of theframing bit.

Inband Signaling: Signals transmitted within the same channel andfrequency bank used for message traffic. See also robbed-bit signaling.

Interface: A common boundary between two systems or pieces of equipment.

isochronous: Recurring at regular intervals. A signal is said to beisochronous if the time interval separating any two significant instantsis theoretically equal to the unit interval, or to a multiple of theunit interval.

Jitter T1.403-1989!: Short-term variation of the significant instants ofa digital signal from their ideal positions in time. Short-term impliesthat these variations are high frequency (greater than 10 Hz).

Kilo Bits Per Second (kbps): 1000 bits per second.

LBO Line Build Out: A cable simulation circuit designed to reproduce thesignal attenuation of a specific length of cable.

LBS Least Significant Bit: The position within a byte that has thesmallest value.

LED (light emitting diode): A semiconductor device that produces lightwhen voltage is applied. LED's provide a visual indication of theoperational status of hardware components, the results of maintenancetests, and the alarm status of circuit packs, and the activation oftelephone features.

Line Alarm Indication Signal (AIS) Code T1.105-1988!: A Line AIS code isgenerated by a regenerator upon loss of input signal or loss of frame.The Line AIS signal will maintain operation of the downstreamregenerators and therefore prevent generation of unnecessary alarms. Atthe same time, data and orderwire communication is retained between theregenerators and the downstream Line Terminating Equipment (LTE).

line code A system of pulses that represents data during transmission.For examples see AMI, BNZS.

Line Loopback T1.403.1989!: A loopback in which the signal transmittedbeyond the loopback point (the forward signal), when the loopback isactivated, is the same as the received signal at the loopback point.

LLB Line Loop Back: A signal path timing mode in which a signal isretransmitted exactly as it is received by the terminal equipment.

LOC Loss of Carrier: a condition of having no information bearingsignal.

Local Access and Transport Area (LATA) T1.502-1988!: A geographic areaestablished for the provision and administration of telecommunicationsservices. A LATA encompasses one or more exchanges that have beengrouped to serve common social, economic, and other purposes.

Local Exchange Company (LEC): A company franchised to provide publicintra-LATA (local access and transport area) telephone service tosubscribers within a defined geographical area. Also called a localexchange carrier or local telephone company.

LOF Loss of Frame: An extended period of the Out of Frame (OOF) state.See also OOF.

Longitudinally Balanced T1.505-1989!: The longitudinal-to-metallic(s-to-m) balance coefficient is expressed as:

Longitudinal Balance (dB)=20 log |Vs/Vm|, Where Vs is the longitudinallyapplied voltage, Vm is the resulting metallic voltage produced by theimbalance of the interface connection. The expected values are:

    ______________________________________                                        Frequency Range      Standard Value                                           ______________________________________                                        20 to 250 Hz         >72 dB                                                   251 to 7,000 Hz      >66 dB                                                   7,001 to 20,000 Hz   >60 dB                                                   ______________________________________                                    

loop activate: A code that, when sent to a remote piece of equipmentsuch as CSU, causes the equipment to go into a loopback condition.

loop code: A specific bit sequence, or code, that tells a far-end pieceof equipment such as a CSU to activate or deactivate its loopback mode.See also loopback.

loop deactivate: A code that, when send to a remote piece of equipmentsuch as a CSU, causes the equipment to go cancel any existing loopbackcondition.

loop down: See loop deactivate.

loop timing: A signal path and timing mode in which the transmitted datais generated or regenerated by the terminal equipment, but the transmittiming is derived from the input signal.

Loopback T1.403-1989!: A state of a transmission facility in which thereceived signal is returned towards the sender.

LOS Loss of Signal: a state declared when there is no detectable signalwhich meets specified parameters.

Mbit/s, Mbs: Notation for "megabits per second". Standard unit ofmeasure for transmission rate. Alternate form: "Mbs." See also megabit.

Mhz Megahertz: One million Hertz, or one-million cycles per second. Seealso Hz.

Modem: A device that converts digital data signals to analog signals fortransmission over telephone circuits. The analog signals are convertedback to the original digital data signals by another modem at the otherend of the circuit.

Most significant Bit T1.502-1988!: The leftmost bit position. Bit 1 asillustrated in FIG. 3, page 37 of designated document.

MU-255: A type of code by chic analog signals are encoded to digitalsignals.

multiframe: A set of consecutive frames in which the position of eachframe can be identified by reference to a multiframe alignment signal.

multiplexer (mux): Technically, a device that performs multiplexing.However, common usage has broadened the definition to include devisesthat perform both multiplexing and demultiplexing. See also muldem.

Multiplexer: A device used to combine a number of individual channelsinto a common bit stream for transmission.

Multiplexing: A process whereby a transmission facility is divided intotwo ore more channels, either by splitting the frequency band into anumber of narrower bands or by dividing the transmission channel intosuccessive time slots. See also time-division multiplexing.

multiplexing: The process of combining multiple signals into a single,higher rate signal for more efficient transmission. Compare withdemultiplexing. See also multiplexer and muldem.

mux: See multiplexer.

mV: Notation for "millivolt:" one thousandth of a volt.

mW: Notation for a "milliwatt:" one thousandth of a watt.

Network T1.403-1989!: A collection of transmission and switchingfacilities used to establish communication channels.

Octet T1.206-1988!: A group of eight binary digits operated upon as anentity.

Off Premises: Telephones or data terminals that are not located withinthe same building or campus as a communications system or have looplengths greater than 3500 feet.

Off-Hook: A condition in which the handset of a telephone is off theswitchhook or the telephone is activated by other means, such as througha speakerphone button.

one: A binary digit, represented digitally by a non-zero pulse. Alsocalled a mark. See also bit.

OOF Out of Frame: also called "Out of Frame Synchronization:" A state inwhich the digital terminal or test set cannot detect a valid framealignment sequence.

PABX Private Automatic Branch Exchange: see PBX.

Parity: A method of checking and, in some cases, correcting the accuracyof bits.

Pattern: For Automatic Alternate Routing--Automatic Route Selection(AAR-ARS), a series of trunk groups arranged in a preferential order.Also called routing pattern.

PCM Pulse Code Modulation: The sampling of an analog signal into abinary code to be digitally transmitted. Common speech digitizing uses8-bit samples and a sample rate of 8 kHz.

Physical Layer T1.110-1987!: The layer (Layer 1) that providestransparent transmission of a bit stream over a circuit built from somephysical communications medium.

plesiochronous: Closely matched in time or frequency: Two signals areplesiochronous if their corresponding significant instants occur atnominally the same rate. For example: two signals having the same bitrate but whose timing comes from separate clocks may be considered.

Private Branch Exchange: See PBX.

protocol: In telecommunications, a formal set of conventions governingthe format and relative timing of message exchange between communicationprocesses.

PSTN Public Switched Telephone Network: Any common carrier network thatprovides circuit switching between public users.

Public Network: A network that is commonly accessible for local orlong-distance calling.

Pulse Density T1.403-1989!: A measure of the number of "ones" (marks,pulses) in relation to the total number of digit time slots transmitted.

Pulse-code Modulation: An extension of pulse-amplitude modulation (PAM)in which carrier signal pulses modulated by an analog signal, such asspeech, are quantized and encoded to a digital, usually binary, format.See also pules-amplitude modulation.

Quasi-Random Signal (QRS) T1.201-1989!: A signal consisting of a bitsequence that approximates a random signal.

RBS Robbed Bit Signaling: In PCM, a scheme in which the signaling bitsfor each channel are assigned to the LSB (Bit 8) or Frames 11 and 12 SFformat, or Frames 6,12,18, and 24 in ESF format.

Red alarm: Indicates a local failure of a carrier system in the receive,or near-to-far, direction. Also called a "local alarm."

Remote alarm: a Yellow alarm (see also Yellow alarm).

Robbed-Bit Signaling: Digital signal level-1 (DS1) signaling in which upto eight kbps from each of the 24 64-kbps channels are used forsignaling in every sixth frame. The least significant bit of each 8-bitsample is replaced by a signaling bit. Also called DS1 robbed-bitsignaling.

signaling bits: Overhead (or robbed) bits that carry dialing and controlinformation on a signal.

slip: The occurrence of a digital signal buffer overflow or underflow ina synchronous digital network. See also controlled slip and uncontrolledslip.

SuperFrame Format (SF, D4): A framing format which allows for 24channels on a DS1 signal. Also called "D4."

Switch: Any kind of telephone switching system. See also communicationssystem and ESS.

synchronization: The state or action of having a common rate(frequency): Two or more signals are synchronized if they run at thesame clock rate.

Synchronous T1.105-1988!: The essential characteristic of time-scales orsignals such that their corresponding significant instants occur atprecisely the same average rate.

T1 Digital Carrier: A type of digital transmission medium that transmitsat 1.544 Mbps and is capable of carrying 24 channels.

T1 Line T1.403-1989!: A full duplex digital transmission facility thatis composed of two twisted metallic parts and regenerators that carryone DS1 signal.

T1: A digital transmission standard that in North America carriestraffic at the digital signal level-1 (DS1) rate of 1.544 Mbps.

T1: The common expression for DS1 (see also DS1). "T1" also refers tothe actual cables or media that carry the DS1 signal.

test pattern: A known, non-random bit sequence used for testingend-to-end or round-trip performance. A received test pattern can becompared to the transmitted test pattern to determine if any errors haveoccurred.

Tie Trunk: A dedicated telecommunications channel connecting two privatecommunications systems. Also called automatic tie trunk.

Tip and Ring: Tip and ring are common nomenclature derived from old cordswitchboard technology to differentiate between the two leads of ananalog line or trunk.

Trunk: A dedicated telecommunications channel between two communicationssystems or central offices (CO's). See also facility.

Twisted Pair: Two copper wires used for the transmission of voice and/ordata.

VF Voice Frequency: Describing an analog signal with a frequency lyingwithin the part of the audio range reserved for speech transmission,approximately 200 to 3500 Hz.

Wander T1.403-1989!: Long-term variations of the significant instants ofa digital signal from their ideal positions in time. Long-term impliesthat these variations are of low frequency (less than 10 Hz).

wink: A return pulse that acknowledges a line seizure or other lineevent before the transmission or reception of dial pulse digits or othercalling information.

Yellow alarm: Also called a "Remote alarm", a Yellow alarm indicates aone-directional failure in the transmit or near-to-far direction.

zero A binary digit: Also called a "space.: See also bit.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a reliable andsimplified T1 channel bank control process and apparatus using minimalcircuit components. Another object of the present invention is toprovide "battery feed" using a small transformer to analog or digitaltelephone lines for the purpose of powering terminating equipment orinterfaces at the far end of a wire line. Applications are in wire-linetelephony such as central office or exchange subscriber lines,multiplexing equipment, and Foreign Exchange Station (FXS) equipment.Very small, low-power, telephone Subscriber Line Interface Circuits(SLIC's) may be constructed using the new technique. Line currents areprovided from current regulators that do not feed through an AC couplingdevice, such as a transformer. The technique provides excellentLongitudinal-to-Metallic balance when measured to standards such as FCCPart 68, IEEE Standard 455-1976, and AT&T TR43801. Longitudinal balanceis required of telephone line feeding circuits to avoid the pick-up ofinduced noise sources, such as power line "hum", in wirelinecommunications circuits. The technique also limits the amount of currentsupplied to telephone lines, thereby decreasing power consumption andheat dissipation in the communications systems.

A DC voltage power source has an output current regulator and an inputcurrent regulator. These two current regulators connect to the wire lineloop which connects to the remote telephone equipment. Each regulator isset to approximately the same current, thereby creating an error currentas the difference between the two current settings. A capacitor is usedto couple the wire line loop to the AC coupling device which in turnconnects to the telephone line. This circuit arrangement createsexcellent longitudinal balance performance and immunity to induced noisesources such as the AC coupling device hum.

Another object of the present invention is to provide a technique forimproving attenuation/frequency distortion and return loss (impedancematching) of transformer-coupled wire-line communications circuits byusing secondary series capacitance and an AC current-pump signal source.

This electronic technique is used for coupling AC communications signalsto analog or digital telephone lines. Applications are in wire-linetelephony such as central office or exchange subscriber lines,multiplexing equipment, and Foreign Exchange Station (FXS) equipment.Very small, low-cost, telephone Subscriber Line Interface Circuits(SLIC's) may be constructed by using an AC coupling transformer that issmall in size and relatively low in magnetic inductance, in comparisonto traditional techniques.

The technique compensates for a low value of transformer inductance byusing a series capacitance in the secondary side of a transformercircuit. The capacitor allow the total circuit to maintain a constantmatching impedance (Zm) as a function of frequency. This results insignificant improvement in Return Loss (minimization of reflectedsignals) in comparison to circuits with no secondary capacitancecompensation.

In addition to the secondary capacitance used for impedancecompensation, a voltage-controlled AC current pump is used to drive thecommunications signal into the secondary side of the transformercircuit. The current pump serves to compensate for the effect of thesmall transformer inductance and low coupling efficiency that wouldotherwise decrease the amplitude of low frequency signals with respectto the amplitude of high frequency signals. The technique avoids animpairment in signal gain as a function of frequency, commonly calledAttenuation/Frequency Distortion.

The AC current pump is constructed to automatically charge its drivevoltage to maintain a constant total AC drive current. To compensate forthe reduction of the transformer impedance Z_(s) the value of the seriescapacitor C is chosen to series resonate with the transformer secondary.The resonance reduces the impedance of the transformer branch,increasing the current through that branch, maintaining an overall flatcircuit loss for the communications signal as a function of frequency.

Because the AC current pump presents a very high impedance to signalsoriginating from the wire line, the termination impedance of the circuitcan be adjusted to match the wire line or connected communicationsequipment independently of the signal source impedance.

Another object of the present invention is to provide for the generationof ringing voltage as positive voltage pulses with respect to a negativepower supply voltage.

A modern circuit for the purpose of operating a telephone must meet awide range of safety and performance criteria. Standards call for thegeneration of relatively high voltage pulses to operateelectro-mechanical devices such as bells. Commonly-provided ringingsignals in North America have a voltage of 85 Vrms and a frequency of 20Hz. Factors affecting the design of a circuit meeting these standardsare:

A. Crest Factor--Ringing pulses must be shaped to a range of"crest-factor", the peak waveform value relative to the rms value shouldbe between 1.2 to 1.6.

B. Output Impedance--The source must present relatively high outputimpedance (about 1400 ohms), for reasons of safety.

C. Ringing Pulse Frequency--The frequency of pulses presented to thetelephone may range from a low of 12 pulses per second to a high of 33pulses per second.

D. Alerting Device Load--Telephone bells or alerting devices aretypically capacitively coupled across the tip and ring conductors ofphone lines. The impedance load presented to the ringing voltagegenerator may only be as high as tens of thousands of ohms, to as low as1400 ohms. The load presented to the ringing generator may be highlyreactive, and the effects of these loads must be accounted for.

E. Peak Voltage Maximum--The voltage may not peak over 200 volts.

F. Momentary Short Circuits--Logic dictates that the circuit be capableof withstanding a short circuit across its output terminals.

G. Telephone Hook Switch Detection--Loop current sensing circuitry mustdetect when the telephone is answered. In order to do this the circuitmust operate at negative telephone battery, below ground. This is toallow loop current sensing circuitry to detect off-hook of a telephone,even during cadence off (when ringing voltage is not present), or whenan off-hook is detected during cadence on when ringing voltage Bpresents.

H. Noise and Electromagnetic Compatibility Issues--When a telephone isanswered, a noise inducing current pulse may be generated. This pulse,if not atttenuated will be picked up in adjacent telephone lines. Thisis because the ringing voltage generator load is no longer only thebell, or similar device, but the relatively low impedance of thetelephone circuit itself (which may be in the 50 to 300 ohm resistiverange). While generating these pulses there must not be enough RadioFrequency radiation to cause the system using the ringing circuit tofail FCC Part 15 radiation levels.

I. System Reliability in the Presence of Circuit Failure--Should somecomponent of the Ringing Pulse voltage generator fail, the circuitshould not draw sufficient current to cause the rest of the system tofail. A circuit failure generating excessive peak voltage must not beallowed to place excessive voltage on the generator output, nor may thatfailure cause successive failures of down stream circuitry.

J. The voltage out of the ringing generator must be about 86 volts rms,over a specified load. The technique of this invention uses a push-pullswitching voltage regulator circuit (FIG. 5) switch on and off at thedesired ringing voltage pulse rate. The ringing voltage pulse rate isgoverned solely by a digital frequency generator. The voltage regulatoris turned on and off by the digital frequency generator's output. Thisfrequency may be varied over the needed range by adjusting frequencycontrolling elements. The output of the digital frequency generator isalso fed to a switch (S1) on the output of the regulator. The operationof this switch is such that when the voltage regulator is on, the switchis open. When the regulator is not generating voltage, the switch isclosed, discharging any capacitance across the output terminals.

A push-pull switching voltage regulator operates at a frequencythousands of times that of the ringing voltage pulse rate. Two outputswitches are alternately operated Open and Closed to apply analternating high frequency current through a small transformer. Thevoltage applied across the transformer's primary winding is multipliedby the turns ratio of the transformer, which boosts the circuit's supplyvoltage by many times. The Transformer is wound to provide the properoutput impedance for the circuit, under maximum load conditions.

A voltage divider feed back circuit incorporated into a pulse widthregulator circuit causes the voltage to be limited to approximately 180volts peak, (or approximately 86 volts rms). The waveform is a shapedrectangular wave at the desired ringing pulse frequency.

The push pull circuit technique is used to minimize the RF radiationfrom the high frequency switching circuit. The voltage doubler action ofthe push-pull circuit also tends to minimize transformer turns ratios,improving the efficiency of the circuit.

Circuit overload protection is provided by a current sensor, operatingin concert with the Pulse Integrator circuit. Each output pulse hasapproximately the same peak current level. The current sensor applies avoltage proportional to the width and amplitude of the peak current tothe pulse integrator. If a sufficient quantity of pulses are applied tothe integrator in a time period, the over current shutdown function ofthe circuit will limit the quantity of pulses output. When the over loadcondition is removed, the circuit operation will return to normal, andthe dc output of the pulse integrator circuit will return to anon-current limiting level.

If the differentiator and voltage feedback circuit were to open,excessive voltage would appear at the generator's output terminals. Anovervoltage protector will short to a very low impedance within a fewmicroseconds of detection of the voltage being over approximately 240volts. This will lower the output voltage to only a few volts over thenegative voltage supply to the circuit.

Another object of the present invention is to provide for the removal ofAC power ripple by using an active linear floating filter for thepurpose of powering telephone line circuits.

This electronic technique attenuates AC ripple voltages from rectifiedtransformer power supplies by factors up to 100, without the use oflarge capacitances. Unlike conventional fixed voltage linear regulators,the output voltage of the circuit floats with respect to the absolutevalue of the rectified AC input source. That is, a 10% increase in theinput source will produce approximately a 10% increase in the outputvoltage. Conventional fixed voltage linear regulator power suppliesproduce an output voltage that remains the same, regardless of inputsource changes.

The purpose of this new circuit is to provide a filtered DC voltage forpowering telephone Subscriber Line Interface Circuits (SLICs). Onefunction of SLICs is to provide a DC line current to terminatingequipment at the other end of the line, for example telephone sets. TheDC line current must be substantially free from AC noise sources, suchas 50 or 60 Hz ripple, that would be heard by telephone users. Byselecting a SLIC circuit with a reasonably wide input voltage range,this new technique provides noise-free current to the telephone linewithout large component size, cost, and power dissipation inherent inusing a fixed voltage-regulated power source.

Conventional fixed voltage-regulated linear SLIC power sources dissipatea significant amount of power and heat. This is necessary to accommodatethe typical range of plus or minus 20% input voltage fluctuations fromcommercial power mains and step-down transformers. The new power filtercircuit described here maintains a constant DC voltage drop rather thana constant DC voltage output.

Still another object of the present invention is to provide for theinjection of real time tone samples into T1 transmission circuits by useof a T1 framer idle code register.

This software technique is used to insert Pulse Code Modulation (PCM)tone samples into a T1 digital time division (TDM) multiplextransmission line at an 8 kHz rate. This technique makes use of the IdleCode Register commonly incorporated in T1 Framer or T1 Controllerintegrated circuits to insert a new tone sample into one or more of the24 channel time slots of a T1 frame in real-time. The Idle Code Registerfunction of such integrated circuits is designed and documented for thepurpose of inserting a constant or quiet (idle) PCM sample in one ormore channels of the T1 frame. Dynamic (8 kHz) updates of the Idle CodeRegister for the purposes of digital tone generation are undocumented todate.

By updating the PCM tone sample from a table of values at every framepulse (occurring every 125 us), simple sinusoidal or complexmulti-frequency (such as ringback tone) digital tone generation isaccomplished. This software tone generation technique makes use of amicrocomputer to write a new tone sample to a single Idle Code Registerof a T1 Framer or Controller in synchronization with the frame pulse ofthe T1. Unlike other techniques, no electronic memory buffer ordedicated register file is used to store and update the PCM tonesamples. The T1 Framer or Controller integrated circuit inserts the IdleCode Register value in the T1 stream at any or all of the 24 channelsthat have been marked to receive the Idle Code in place of PCM samplescoming from other sources in the communications system. Tone generationis thus done by real-time microcomputer software updates and the IdleCode substitution capabilities of the T1 Framer or Controller integratedcircuit.

Revolutionary Integration

The present invention sets a new standard for connecting T1 switchedservices to Hybrid/Key and PBX telephone systems. The present inventionknown as the Access Bank™ converts a T1 digital access line to 12 or 24individual analog telephone circuits. The Access Bank™ provides the mosteconomical, compact, and reliable solution on the market for convertingT1 digital access services from AT&T, LDDS, MCI, Sprint, and localtelephone companies to standard "dial tone" interfaces. Business andgovernment offices can connect their existing telephone systems to T1digital services--without a significant investment in equipment.

Using state-of-the-art solid state integrated circuits,ultra-reliability and advanced service features have been engineeredinto the Access Bank™. Only three circuit cards make up a complete 24channel bank. The FCC-registered T1 Channel Service Unit (CSU), RingingGenerator, Power Converters, Ringback Tone Generator, and Channel BankController are all integrated into a single small electronics cardcalled the Line Interface Unit (LIU). The LIU connects directly tocarrier T1 lines and supports selectable T1 standards of D4 or ESFframing and AMI or B8ZS line coding. A quick field replacement of theLIU completely changes all common electronics in the Access Bank™.

Intelligent Compatibility

One or two 12 channel telephone line voice cards plug into the LIU cardto make a complete Access Bank™. The most commonly used 12-channel voicecard is the FXS-12. One FXS-12 card provides 12 standard Loop Start orGround Start telephone line connections to a customers telephone system.The second FXS-12 card (for 24 telephone lines total) can be added tothe Access Bank™ simply by sliding it into the back of the cabinet. TheFXS-12 cards contain their own intelligent microcontrollers, providingthe following signaling conversions (selectable) for major carrier T1services:

Standard FXS Loop and Ground Start signaling for MCI, Sprint, LDDS, andmost local telephone operating companies.

E&M Wink Signaling on the T1 line can optionally be converted to astandard Group Start or Loop Start connection to a customer's telephonesystem, for example

AT&T Megacom® (ringback tone provided by the Access Bank™)

Nynex FlexPath®(with ringback tone provided by the Access Bank™.

Options for the delivery of Automatic Number Identification (ANI) andDirectory Number Identification (DNIS) 800 services are built in.

New Standards of Service, Safety, and Support

In addition to dramatic functional integration and economy--the AccessBank™ incorporates several new innovations in channel bank andmultiplexer design:

A front-panel Self Test Switch automatically disconnects the AccessBank™ from the T1 line and performs a self diagnostic test. When SelfTest is activated, each FXS channel also provides aninternally-generated ringback tone to the telephone line. This feature,plus a ringing output test for each channel, verifies channelfunctionality independently of the T1 line or remote-end switchconditions.

Solid state lightning and overcurrent protection are sued on all voicelines and the T1 line. The Access Bank™ complies with NationalElectrical Code and UL 1459 requirements for the safety of equipmentattached to telephone wiring. All line attachments withstand 600 Vrmspower connections without fire hazard. Using new electronic technology,the Access Bank™ obtains safety compliance without anyfuses--dramatically improving long-term reliability.

Other objects of this invention will appear from the followingdescription and appended claims, reference being had to the accompanyingdrawings forming a part of this specification wherein like referencecharacters designate corresponding parts in the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a communications network having the presentinvention as one component.

FIG. 2 is a circuit diagram of a telephone Subscriber Line InterfaceCircuit having an unbalanced current source and current sink.

FIG. 3 is a circuit diagram of a transformer, coupled wire linecommunication circuit having a secondary capacitance and an AC currentpump signal source.

FIG. 4 is a circuit diagram of a ringing voltage generator having apositive voltage pulse with respect to a negative power supply voltage.

FIG. 5 is a block diagram of the ringing voltage generator of FIG. 4.

FIG. 6 is a graph of the ringing voltage output of the generator shownin FIGS. 4, 5.

FIG. 7A and 7B are a circuit diagram of a telephone filter line havingan active linear floating filter.

FIG. 8 is a schematic of a real time tone sample injector.

FIG. 9 is a schematic of the Line Interface Unit (LIU) and TwelveChannel Line Card.

FIG. 10 (prior art) is a circuit diagram of a traditional battery feedtechnique.

FIG. 11 (prior art) is a circuit diagram of a transformer hybrid.

FIG. 12 (prior art) is a circuit diagram of an electronic hybrid.

FIG. 13 (prior art) is a circuit diagram of a transformer impedancecompensation technique.

Before explaining the disclosed embodiment of the present invention indetail, it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangement shown, sincethe invention is capable of other embodiments. Also, the terminologyused herein is for the purpose of description and not of limitation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A "battery feed" power source is required, for use by telephoneterminating equipment or interfaces at the far end of a wire line. Asshown in FIG. 2, the unbalanced current source and current sink 100provides a regulated current I_(LOOP) to the wire line 101 via theconnections to the line 100, TIP and RING.

The AC coupling device 106 interfaces communication signals S₁ betweenthe wire line 101 and the electronic communication system 107. The ACcoupling device 106 terminates communication signals with R_(TERM) andC_(TERM), typically 900 ohms and 2.16 microfarads to match the impedanceof the wire line 101. Capacitor C_(TERM) blocks DC current flow from theunbalanced current source and current sink 100 into the AC couplingdevice 106. Thus, the size of the AC coupling device 106 is decreasedcompared to a traditional coupling device that must handle DC currents.Since the AC coupling device 106 provides a matching termination to thewire line, the source impedance of the unbalanced current source andcurrent sink 100 must be much higher than the terminating impedanceprovided by the AC coupling device 106.

Battery 105 is a low-impedance source for current I_(LOOP) to the wireline 101. The unbalanced current source and current sink 100 mustmaintain at least a minimum potential of V_(REG),MIN. Maintaining aminimum potential V_(REG),MIN. that the required high source impedanceis provided by each current regulator, REG_(T) and REG_(R),. To meetrequirements of providing I_(LOOP) over a range of load resistancepresented across TIP and RING by the wire line, each current regulatormust maintain a maximum potential no greater than V_(REG),MAX. Common,inexpensive semiconductor current sources alone are not adequatelymatched to meet these voltage constraints. Prior implementations to feedI_(LOOP) from a high-impedance current source have usedexpensive,precisely matched (current mirrored) regulators for REG_(T)and REG_(R).

The unbalanced current source and current sink 100 uses resistors R_(T)and R_(R) to compensate for variations between regulators REG_(T) andREG_(R) (ie. I_(T),REG does not exactly equal I_(R),REG), so thatinexpensive regulators can be used. Specifically, R_(T) and R_(R) arechosen to assure that

    I.sub.LOOP =I.sub.T,REG +I.sub.T,RES =I.sub.R,REG +I.sub.R,REG

within the constraints on V_(REG),MIN and V_(REG),MAX. To accomplishthis if R_(T) =R_(R) =R and if the maximum difference between I_(T),REGand I_(R),REG is I_(DIFF),MAX, then R is chosen to meet

    V.sub.REG,MIN /R+I.sub.REG,MAX =V.sub.REG,MAX /R+I.sub.REG,MIN

or

    R=(V.sub.REG,MAX -V.sub.REG,MIN)/(I.sub.REG,MAX -I.sub.REG,MIN)

For example, the above calculation using typical values for V_(REG),MAX(6.0 volts), V_(REG),MIN (3.0 volts), I_(REG),MAX (28 mA), andI_(REG),MIN (27 mA) gives 3.0 kohms for R. The value of R_(T) and R_(R)can be decreased from this value to account for resistor tolerances.

Together, the unbalanced current source and current sink 100 and the ACcoupling device 106 provide excellent longitudinal-to-metallic balancewhen measured to standards such as FCC Part 68, IEEE Standard 455-1976,and AT&T TR43801. Good longitudinal balance is required to avoid thepick-up of induced noise sources, such as power line "hum," in wire-linecommunications circuits. The unbalanced current source and current sink100 provides a very high impedance to both metallic and longitudinal ACsignals. For longitudinal signals from wire line 101, R_(T) appears inseries with TIP, and R_(R) appears in series with RING. In our exampleabove, with R_(T) =R_(R) =3.0 kohms, longitudinal balance is 60 dB witha mismatch of 1% between the resistors. This is an improvement of 8 dBcompared to the balance obtained with traditional 450 ohm resistorsmatched to 1%. Since the AC coupling device 106 does not carry any DC,it has no path to circuit ground, and does not contribute anylongitudinal-to-metallic signal conversion.

The telephone line interface analog circuitry utilizes an electronichybrid circuit comprised of A₁, A₂, A₃ and A₄ as shown in FIG. 3. Thereceived voice band signal received from the carrier system drives thenon-inverting input of A₁. The gain of A₁ is controlled with three SPSTswitches S905A, S905B and S905C giving a range of control by voltage,dividers R942, R923 and R924 and change of gain by feedback resistorsR920 and R921. The output of A1 drives the inverting input of A₂ and thenon-inverting-input of A₃. Amplifier A₂ is configured as a HowlandCurrent Pump. The output of the circuit is at the junction of R929 andR930 and connected to the balancing network C904 and R912 across thesecondary of T901 in series with C903. The Howland Current Pump deliversa constant current to these components if the value of R₉₃₀ =(R₉₂₇/R₉₂₅) ·R₉₂₉ -R₉₂₈. When this condition is met, the negative resistanceoffered by R₉₃₀ by the positive feedback of A₂ equals the positiveresistance of R₉₂₉ +R₉₂₈ resulting in an extremely high source impedanceto the output current being delivered to the transformer secondary andbalance network. The value of the current is determined by the voltageat the output of A₁ and a resistive network comprised of R₉₂₇, R₉₂₅ andR₉₃₀ and is:

    I.sub.z =-V.sub.in ·R.sub.927 /R.sub.925 ·R.sub.930).

The above constant current Iz divides between T901+C903 and R₉₁₂ inparallel with C904. The current to the transformer T901 and capacitorC903 being the total current I_(L) due to the output of A₁ and theresistive network multiplied by the impedance of the balance network anddivided by the sum of impedance of the balance network and thetransformer-capacitor branch.

     (V.sub.in ×R.sub.927)/(R.sub.925 ×R.sub.930)!× Z.sub.Bal /(Z.sub.Bal +Z.sub.Trans)!

At the low end of the voiceband around 500 to 1000 Hz the impedance ofthe transformer recedes because of the relatively small inductance ofthe miniature transformer. Typical values of inductance for these smalldevices is 0.5 Henry. By placing C₉₀₃ in series with this transformerinductance, a resonant frequency of about 300 Hz may be achieved. Atthis frequency the impedance of the branch is minimum and in accordancewith the above formula the maximum current flows through T₉₀₁ improvingits response at the low end of the band.

The electronic hybrid circuit consists of A₂ and A₃. The voltage createdby the constant current output of A₂ and its resistor networks producedby the parallel combination of balance network and the transformerbranch appears at the output terminal 1 of A₂. This voltage is invertedfrom the output of A₁ by the current pump A₂. The output of A₁ is alsoconnected to the non-inverting input of A₃ whose gain and phase arecontrolled by the feedback network on the inverting input. The purposeof R₉₃₅, R₉₃₆ and C₉₀₇ are to adjust the gain and phase of the output ofA₁ to cancel its inverted image at the output of A₂ in the summingresistors R₉₃₁ and R₉₃₄ at the input of A₄.

Also present in the output of A₂ is the non-inverted image of thereceived signal from the transmission line. A₄ passes this receivedsignal, inverted and slightly diminished to a voltage divider networkadjusted by switch S905F. Additional attenuation is provided by S905Dand S905E and the action of TP3054N.

A ringing generator function is accomplished by creating positivevoltage pulses with respect to a negative power supply voltage.

Description

The following description includes references () to the above designfactors. The design meets the design factors referenced.

The circuit (410) shown in FIG. 5 meets these somewhat conflictingcriteria:

A push-pull switching voltage regulator circuit (U2) is switched on andoff at the desired ringing voltage pulse rate (Factor C). The ringingvoltage pulse rate is governed solely by U1, the digital frequencygenerator. The voltage regulator is turned on and off by U1's output.This frequency may be varied over the needed range by the high frequencycontrol element, which could be a resistor/capacitor, ceramic resonator,or crystal. The output of the U1 is also fed to a switch (S1) on theoutput of the regulator. The operation of this switch is such that whenthe voltage regulator is on, the switch is open. When the regulator isnot generating voltage, the S1 is closed, discharging any capacitanceacross the output terminals.

(Factor D)

The push-pull switching voltage regulator operates at a frequencythousands of times that of the ringing voltage pulse rate, allowingenergy control with smaller circuit elements. Output switches S2 and S3are alternately operated Open and Closed to apply an alternating highfrequency current through the transformer T1. The voltage applied acrossT1's primary winding is multiplied by the turns ratio of the Transformerwhich boosts the circuit's supply voltage by many times. (Factor J) TheTransformer is would to provide the proper output impedance for thecircuit, under maximum load conditions. (Factor B). The transformer T1multiplies the supply voltage about 8 times. This high frequency ACenergy is delivered to the full wave bridge (U7) to produce ringingpulses of a singular positive polarity of approximately 180 volts peak.

The pulse peak output of U7 is summed on top of the--Vin voltage node.This--Vin voltage source is current limited by U3, U3 contains circuitryto limit noise spike current and large heating current due to normallycatastrophic events such as high voltage power cross.

A voltage divider feed back circuit incorporated into U5 causes thevoltage to be limited to approximately 190 volts peak, (or approximately86 volts rms., the waveform being a shaped rectangular wave at thedesired ringing pulse frequency). (Factor E) The speed of the turn on ofthe voltage generator (U2) is governed by the differentiator circuit ofU5. Thus the crest factor required by some specifications may be set byU5 (Factor A).

The push pull circuit of S1/S3 is used to tend to minimize the RFradiation from the high frequency switching circuit. The voltage doubleraction of the push-pull circuit also tends to minimize transformerratios, improving the efficiency of the circuit. The symmetrical natureof the switching of the push-pull circuit tends to minimized RFradiation from the circuit. (Factor H). The construction of thetransformer using pot cores with a plastic bobbin insert reduces RFradiation. The circuit may also be constructed of a Toroidal Coretransformer, minimizing electrmagnetic radiation.

Circuit overload protection is provided by a current sensor, U8operating in concert with U4, the Pulse Integrator circuit. Each outputpulse is approximately the same peak current level, and the currentsensor applies a voltage proportional to the width and amplitude of thepeak current to the pulse integrator U4. If sufficient quantity ofpulses are applied to the integrator in a time period, the Over currentshutdown pin of U2 circuit will limit the width and quantity of pulsescontrolling S2 and S3. When the over load condition is removed, thecircuit operation will return to normal, and the DC output of the PulseIntegrator circuit of U4 will return to a non-current limiting (lower)voltage level. (Factor F).

If the differentiator Shaper circuit U5 were to open, excessive voltagewould appear at the generator's output terminals. Overvoltage ProtectorU6 will short to a very low impedance within a few microseconds ofdetection of the voltage being over 240 volts, approximately. This willlower the output voltage to only a few volts over the negative voltagesupply to the circuit. (Factor E and Factor I)

FIG. 4 is the ringing generator circuit (400). Power to the ringinggenerator circuit (400) is isolated, at least from short conditions, byPolySwitch® R19, a Raychem RXE050. The device referred to as aPolySwitch® is a positive temperature coefficient resistor. When thecurrent rises above a predetermined level the PolySwitch® will increaseits resistance by several orders of magnitude. Incorporated herein byreference are the following U.S. Pat. Nos. 4,237,441 4,238,812 4,413,3014,475,138. This device has a trip current of 1 amp and a holding currentof 1/2 amp. Resistor R200 is a 5.6 ohm 2 watt resistor in series withthe Negative 48 volt (N48V) current supply path. Capacitor C5 suppliespeak current to the ringing generator circuit (400) during periods ofhook switch detection. This occurs when the ringing generator relay (notshown) on the FXS card (not shown) is operated and is connecting theRNGVOLTS signal to a particular FXS circuit on that card. When someonepicks up a telephone on a ringing tip/ring pair, the telephone's hookswitch closes, and momentarily a large current flows in the tip/ringcircuit. This peak current could cause noise in the adjacent channels.Resistor/capacitor combination R200/C5 serve to isolate channel servedby the same power supply from the noise present on the RNGVOLTS nodewhen a ringing telephone goes off hook.

The general purpose of the ringing generator is to produce a roundedwaveform shape (FIG. 6, 403) to about 180 V peak (402), riding on top ofthe negative supply voltage (401). This is accomplished by turning onand off a DC/DC step-up converter at an on/off rate of 20 Hz. Thewaveform produced with each turn-on begins with a gradual ramp-up at arate governed by an R-C time constrant (403). At the end of each pulseof the waveform (403), a switch (S5) closes across the generator'soutput to discharge any residual charge remaining on the output. Theload (not shown) presented to the ringing generator is often mainlycapacitive, and S5 returns any residual charge, after the end of aringing pulse, to zero. Resistor R70 limits the peak current through S5,keeping noise inducing current a minimum.

Within the ringing generator circuit (400) two internal voltages areused (generated with FIG. X): A switched 18 volt level (+18V) foroperating the pulse driving circuitry and a +5 Volt level (+RNGRLOGIC)for operating the digital counter circuit (U12). Both this +5v level andthe +18v level are relative to the N48V node. The +18v level is obtainedfrom a zener diode regulator circuit, CR5 (not shown), which is thenrouted through the series solid state switch contacts of U7 (not shown).Switch U7 (not shown) is operated by the comparator U10 (not shown).Comparator U10 (not shown) is part of a circuit that senses the inputpower supply voltage (N48V) node and temperature. If the input DCvoltage level (N48V) is -42 volts are greater, and if the temperature isless than 40 deg.C. then U7 (not shown) is closed, which then applies+18V to the ringing generator circuit (400). The 15 volt level isobtained from a voltage reference regulator which is located inside theswitching regulator integrated circuit, U21.

When voltage is applied to the circuit (400), an internal oscillatorinside of U12 begins operation. U12 is combination oscillator and 14stage ripple counter. The oscillator operates at about 81 kHz (for a 20Hz pulse output frequency). This 81 kHz frequency is set by R74, R73,and C92 and is applied to the input of the ripple counter, locatedinside U12. Component C92, R73, and R74 form frequency control elementswhich may easily be changed, which also might consist of a crystal,ceramic resonator, or similar frequency source. A 20 Hz squarewave isobtained by the down counter at U12 Output QM. This is applied to thebase of Q10, through R30. Resistor R30 limits the amount of base currentthrough Q10, and presents a high impedance to the output of the 20 Hzsource, U12 output QM. The common emitter circuit of Q10 inverts thatphase by pulling down the voltage across R34. This action of Q10provides two logic level signals such that when the regulator chip U21is in the cutoff (non-pulse producing) state, then S5 is closed. Asmentioned earlier, S5 then removes any residual charge from the output.When the 20 Hz signal goes high out of U12 output QM, then S5 opens,allowing the RNGVOLTS node to go to its high voltage level, driven bythe pulses from Transformer T4, across the high frequency diode bridgeformed by CR19, CR20, CR21, and CR25. Capacitor C76 provides a lowimpedance path for the high frequency currents produced by the alternateswitching action of Q4/Q5.

Integrated Circuit U21 is a pulse width modulator circuit. It producespulses to alternately turn-on a pair of transistors whosecollector/emitter pairs are connected to pine 14/13/ and 12/11. Voltageto operate these output pairs comes from jumper J7. J7 is installed aspart of the manufacturing process, to allow manufacturing personnel theoption of turning off the high voltage generator, when not needed.Capacitor C75 provides local filtering of the +18 volts. Positive 5volts to operate the voltage divider string R42/R43 comes from U21pin16. The divider string mentioned provides 2.5 volts to U21 pin 2, asa reference for operation of the voltage regulator. Test pin 23(RNGOSC)allows factory personnel easily check the frequency of the U21'sinternal oscillator. Pulses of 3-5 volts amplitude are output at thistest point. Resistor/capacitor pair R75/C95 are selected to provide aswitcher frequency in the range of 120 to 200 kHz. Resistor/capacitorpair R79/C96 are installed at U21 pin 9 in order to stabilize theoperation of the voltage feedback loop.

Output pulses are formed across the parallel combinations of R80A/B/Cand R65A/B/C to provide pulse width which is proportional to outputvoltage. These pulses are fed to the gates of the MOS transistors Q4/Q5through coupling capacitors C68/C72. Resistors R44/R37 limit the amountof charge across the gate input capacitance of the transistors Q4/Q5.

Integrated circuit U21's internal oscillator operates at about 120-200Khz, producing a push-pull drive to power, MOS FET transistors Q4 andQ5. Each MOS FET alternately turns on, providing a peak current of 1 to2 amps of across the through the primary of T3. The source connectionsof Q4 and Q5 are tied together and connected to the negative 48 voltrail through R38. The pulses appearing across R38 are integrated by D2,C73, R69. The time constant of R69/C73 determines the over current senselevel that will shut down the output of the ringing generator.

The voltage appearing across the center-tapped primary of T4 is steppedup 8 times to provide 400-500 volts peak open-circuit voltage intocapacitor C87. This amount of voltage can never appear across theoutput, because of the high voltage/high frequency pulses are integratedacross the C87. The pulse density is controlled by U21 to regulate theoutput voltage when the circuit is in operation. The output voltage isfed back to the switcher by resistive divider network R66/R60.Capacitors C7 and C8 together with resistor R36 form a differentiatorcircuit across R66 to limit the initial turn-on risetime of the outputof the ringing generator circuit. This action is approximated by theformula

    Vout=Vmax(1-e t(R36)×(C7+C6))

Ringing output overvoltage is protected by SIDACtor® CR15. This devicehas a 240 volt breakover point. Should a lightning strike, high voltagepower cross, or other similar event occur, the circuitry of the ringinggenerator would be protected by CR 15 triggering to a low resistance.The resistance to the SIDACtor® will stay low until the current dropsbelow the minimum holding current of the device (ie. it acts as anautomatically resetable crow bar). If the voltage feedback path to pin 1of U 21 should fail open, a large DC voltage could be placed on theRNGVOLTS node. All of the circuitry connected to the RNGVOLTS node havebreakdown voltage of at least 350 volts. Therefore, the possibility ofdamage by ringing generator circuit (400) itself is protected bySIDACtor® CR15.

Removal is shown of AC power ripple by using an active linear floatingfilter for the purpose of powering telephone circuits. FIGS. 7A and 7Bare schematics of the circuit to remove AC power ripple using an activelinear floating filter.

This electronic technique attenuates AC ripple noise (420) fromrectified transformer power supplies by factors up to 100, without theuse of large capacitances. Unlike conventional fixed voltage linearregulators, the output voltage of the circuit floats with respect to theabsolute value of the rectified AC input source (Vs). That is, a 10%increase in the DC input voltage (401) will produce approximately a 10%increase in the output voltage (Vo). Conventional fixed voltage linearregulator power supplies produce an output voltage that remains thesame, regardless of input source changes.

The purpose of this new circuit is to provide a filtered DC voltage forpowering telephone Subscriber Line Interface Circuits (SLICs) (402). Onefunction of SLICs (402) is to provide a DC line current to terminatingequipment at the other end of the line, for example telephone sets (notshown). The DC line current must be substantially free from AC noisesources, such as 50 or 60 Hz ripple, that would be heard by telephoneusers. By selecting a SLIC circuit (402) with a reasonably wide inputvoltage range, this new technique provides noise-free current to thetelephone line without large component size, cost, and power dissipationinherent in using a fixed voltage-regulated power source.

Conventional fixed voltage-regulated linear SLIC power sources dissipatea significant amount of power and heat. This is necessary to accommodatethe typical range of plus or minus 20% input voltage fluctuations fromcommercial power mains and step-down transformers. The new power filtercircuit described here maintains a somewhat constant DC voltage drop(Vr) rather than a constant DC voltage output (Vo).

FIGS. 7A and 7B show the new technique to power telephone SLICs wherebyan adjustable linear regulator semiconductor circuit (U10) has itsadjust terminal connected to a fixed voltage divider ratio(R74/(R73+R74)) of the input voltage source (Vs). The linear regulatorsemiconductor circuit (U10) has a reference voltage (Vr is typically1.25 Vdc), and amplifiers (not shown), which act to maintain Vr betweenthe In and Adjust terminals. Current Ir=Vr/R73 flows almost entirelythrough the resistive divider (R73 and R74), as very little errorcurrent flows into the high-impedance Adjust terminal. Voltage Vo at theOut terminal of the linear regulator semiconductor circuit (U10) isrelated to the current Ir as, Vo=(Ir)(R74)+Vr.

DC current changes in Ir due to variations in the input source voltageVs cause Vo to change in a linear manner. AC current changes in Ir from50-60 Hz ripple (or higher frequency noise sources) cause very littlechange in Vo since the AC current component of Ir is passed to thereturn node by capacitor C28, and is not allowed to change the voltageat the Adjust terminal (Va). This effectively attenuates the ACcomponent of Vs, as measured at Vo, by a factor as high as 100. ACripple noise is thus effectively removed for the purpose of poweringtelephone Subscriber Line Interface Circuits (SLICs).

Resistor R75 is to provide an output return to ground (BATRET) if thereare no off-hook SLICs (402) drawing current from the circuit.

Capacitators C13 and C31 serve to filter the voltages at the Vin and Voterminals of the linear regulator semiconductor circuit U10. Diode CR26protects the Adjust terminal of the linear regulator semiconductorcircuit V10 from a condition where voltage Va is more negative than theVin terminal. This condition could occur when the source voltage (Vs) isshorted or removed.

If a fault occurs that causes a current greater than 1/2 amp,Polyswitch® S9 will become a high impedance to limit the input current(Iin) until the fault is cleared. Diode CR27 protects the floatingfilter from voltages more negative than Vo, which may originate from thetelephone or SLICs (402). The linear regulator semiconductor circuit(U10) is further protected from either AC or DC over voltage at theinput or output by a 36 volt zener diode CR22 and 110 volt peakSIDACtor® CR25.

Modern systems for T1 digital time division multiplex (TDM) transmissioncommonly incorporate integrated circuits that perform many of thedigital functions required by the T1 specifications. FIG. 8 shows theuse of such a T1 framer/controller integrated circuit 501 in a T1system. Data encoded from the analog voice channels is processed by thetransmit side formatter 502 of the T1 framer/controller integratedcircuit 501 for transmission over the T1 line. Functions of the transmitside formatter 502 include framing bit insertion, signaling insertion,and alarm generation. Similarly, the receive side framer 503 processesthe receive data stream from the T1 line before decoding into analogvoice channel signals. The receive side framer 503 synchronizes theincoming data stream, extracts signaling, and monitors received data fortransmission errors.

A T1 framer/controller integrated circuit 501 may be chosen thatinterfaces directly with a microcomputer 500. In this configuration, themicrocomputer 500 controls operation of the T1 framer/controllerintegrated circuit 501 as well as operation of the communication systemof which it is a component. For example, upon reception of anappropriate signaling state from the T1 line, the microcomputer 500would initiate ringing of the wire line associated with particularchannel. Then, the microcomputer 500 would cause the T1framer/controller integrated circuit to transmit an off-hook signalingstate for that channel to the T1 line when the microcomputer detectsthat the equipment connected to the wire line has gone off hook.

One function of the T1 framer/controller integrated circuit 501 isinsertion of an idle code into the transmit data to the T1 line. Thisallows the microcomputer 500 to replace meaningless data from analogchannels that are not in use with a constant value, so that noise is notpresented to the equipment at the far end of the T1 line. To accomplishthis, the T1 framer/controller integrated circuit 501 incorporates anidle code register 504 that may be programmed by the microcomputer 500.Microcomputer 500 can specify what data to apply as the idle code, andto what channel or channels the idle code should be applied. Then, thetransmit side formatter 502 can insert the idle code into the transmitdata to the T1 line at the appropriate times, using multiplexer (mux)505.

In a channel bank application, it can become necessary to insert tonesinto the data stream transmitted to the T1 line. The tone insertionscheme accomplishes this using only the microcomputer 500 and the T1framer/controller integrated circuit 501. Since no parts are added tothe system for this function, system size and cost are minimized. Unlikea constant idle code that is transmitted identically every125-microsecond frame, tone generation requires a new sample to betransmitted every frame. Unlike older microcomputers that could onlyprocess a few dozen instructions during 125 microseconds, microcomputer500 can be chosen to process hundreds of instructions during a frame. Atthe start of each frame, T1 framer/controller integrated circuit 501alerts microcomputer 500. Then, microcomputer 500 updates idle coderegister 504 with a value for use during the upcoming frame. Softwaretiming causes these updates to occur at the frame boundary, so that idlecode register 504 is not in use during the update.

During the remainder of each frame, microcomputer 500 is available toperform its channel bank control functions. Prior to the start of thenext frame, the next value to be inserted into the idle code registermust be known. Choices for accomplishing this include calculationsand/or look-up tables. To minimize processor loading and memoryrequirements within microcomputer 500, the tone insertion scheme uses acombination of a look-up table and calculation. Because the frequencyspectrum of the implemented tone is sufficiently below the 8 kHz.sampling rate of the data bank that aliasing is not a concern, a look-uptable with values for every second frame rather than every frame isused. For the long pattern length of the multitude combinationimplemented, this saves a considerable amount of memory. To saveprocessor loading, each sample in the look-up table is used twice insuccessive frames. If improved noise performance of the tone had beenrequired, interpolation between look-up table entries could have beenused.

The Access Bank™ (FIG. 9) incorporates three electronic circuit cards.Two identical Twelve Channel Line Cards (1000 and 1010) and one LineInterface Unit (1020) card make up a full 24 channel bank. The LIU cardis mounted in the front of the chassis. The Twelve Channel Line Cardsmount side-by-side in the back of the chassis and are removable on cardguides.

LINE INTERFACE UNIT (LIU)

The ™ (FIG. 9) incorporates three electronic circuit cards. Twoidentical Twelve Channel Line Cards (1000 and 1010) and one LineInterface Unit (1020) card make up a full 24 channel bank. The LUI card(1020) is mounted in front of the chassis (not shown). the TwelveChannel Line Cards (1000, 1010) mount side-by-side in the back of thechassis (not shown) and are removable.

The LIU card (1020) has electrical connectors (not shown) for all lineand power connections to the Access Bank™. The Test/Monitor Jacks (1021)allow the connection of T1 test equipment to monitor the T1 lineperformance. The T1 Digital Carrier Input & Output Connector (1022) isthe FCC-Registered connection point between the Access Bank™ and apublic telecommunications carrier T1 line. Power is supplied to theAccess Bank™ by connection to the Battery Input (1031). An unfiltered-48 Volt dc power source (1050) is supplied for powering the AccessBank™ from 115 Volt ac commercial power.

Twenty four Tip and Ring connections (3037) originate from the twoTwelve Channel Line Cards (1000 and 1010) within the Access Bank™, andare routed to a telephone line connector on the front of the LIU (1020).

Switches on the front of the Access Bank™ LIU (1020) set userconfiguration options and initiate Access Bank™ test functions. They areread by the Microprocessor (1035) firmware that controls the LIU (1020).The Self Test switch (1023) disconnects the Access Bank™ from the T1line and initiates an internal self diagnostic test. A Green Test LED(1027) on the front of the LIU card indicate that the Access Bank™ selftest has passed.

The Network Loopback switch (1024) provides manual activation of theAccess Bank™ CSU function to retransmit the T1 signal received back tothe T1 line.

Option switch (1025) has no currently assigned function. Alarm Cut-Offswitch (1026) turns off the internal Alarm Relay (not shown) connectedto Pair 25 of the telephone line connector(not shown).

Operational status information is provided with multi-color LEDsapparent from the front of the Access Bank™. LED outputs come from theMicroprocessor (1035) firmware that controls the LIU (1020). The TestLED (1027) provides indications for self test results and T1 loopbackstatus. The Status LED (1028) indicates operational status of the LIUcard (1020), including indication of the T1 Trunk Processing State. TheT1 Line LED (1029) indicates the presence of normal (green) or abnormal(yellow) T1 pulses. The Framing LED (1030) provides indications ofnormal or abnormal T1 framing conditions as seen from equipment at thefar-end of the T1 line.

T1 signals from the T1 Digital Carrier In & Out connection (1022) passthrough a Loopback Circuit (1039), which has the ability to connect theinternal T1 Transmit (1037) circuit's signal back to the T1 Receivecircuit (1038). The Loopback Circuit is activated as part of the AccessBank™ Self Test function to test Access Bank™ internal T1 functionality.

The T1 Framer circuit (1036) provides T1 framing, channel formatting,signaling input and output, and T1 alarm management for the Access Bank™LIU. It is configured and controlled by the Microprocessor (1035).

Unfiltered, rectified dc power entering Battery Input connector (1031)passes through monitoring and protective circuitry (1032) beforeproviding power to the Ringing Generator (1033) and DC/DC Converters(1034). The Ringing Generator (1033) produces 85 Volts ac/20 Hz powerwhich is used to ring telephone instruments or telephone systemsconnected to the Access Bank™. The DC/DC Converters originate +5 Voltand -6 Volt supply currents that are used to power electronics on theLIU (1020) and Twelve Channel Line Cards (1000, 1010).

The LIU (1020) connects to each of the Twelve Channel Line Cards byseparate electrical connectors(not shown). Supply power, digitalsignals, and analog signals are passed over each of the two connectorsto the Twelve Channel Line Cards (1000, 1010) via a Parallel Bus(3038)and a Power Bus (3039).

TWELVE CHANNEL LINE CARDS

The Twelve Channel Line Cards (1000 and 1010) each have capability todrive 12 loop-start or ground-start telephone lines (3037). ForeignExchange Station (FXS) interfaces (not shown) provide a standard batteryand ringing voltage interface to telephone instruments, Private BranchExchange trunks, or Key System line interfaces (not shown).

The Coder/Decoder circuit (1004 and 1014) performs both-way conversionof the digital Pulse Code Modulation (PC) samples from the Parallel Bus(3038) to analog voice frequency signals. The analog signals areconditioned by Line Impedance Build Out amplifiers (1003 and 1013).Audio Signal I/O circuits (1002 and 1012) perform amplification andtwo-wire to four-wire hybrid conversion.

High-voltage Isolation Barriers (1001 and 1011) connect the analog audiosignals to the Tip and Ring telephone lines. The Isolation Barrierperforms several of the functions commonly defined for a Subscriber LineInterface Circuit (SLIC). These functions are balanced current feed tothe telephone line, detection of signaling currents, over-voltageprotection, and the application of ringing signals. Opto-electronicdevices in the Isolation Barrier circuits (1001 and 1011) are controlledand read by the Line Card Microprocessors (1041 and 1042) to performsignaling functions. The Line Card Microprocessors (1041) reportsignaling information from each channel of the Access Bank™ to the LIUMicroprocessor (1035), which in-turn reads and writes signalinginformation to the T1 Framer (1036).

Microprocessors (1041 and 1042) also read signaling option settings fromBusy/Test switches (1043 and 1044) on the back panel of the AccessBank™. Channel status indications are output by Microprocessors 1041 and1042 to LED's 1045 and 1046 to show the current signaling state of eachvoice channel in the Access Bank™.

Although the present invention has been described with reference topreferred embodiments, numerous modifications and variations can be madeand still the result will come within the scope of the invention. Nolimitation with respect to the specific embodiments disclosed herein isintended or should be inferred.

We claim:
 1. A ringing voltage generator functioning to provide aringing voltage referenced to a negative voltage output for poweringSingle Line Interface Circuits (SLICs), comprising:a negative voltageoutput; a negative voltage input; a ringing voltage output having aringing pulse functioning to drive the ringing circuits of a pluralityof (SLICs); a digital frequency generator U1 functioning to generate aringing voltage pulse rate; a first frequency control element resistorR1, and first capacitor C1 connected to said digital frequency generatorU1 functioning to set said ringing voltage pulse rate; a push-pullswitching voltage regulator circuit U2 having a regulation input, anON/OFF input, an overvoltage shutdown input, a first driver output, anda second driver output; a transformer T1 having a primary winding with acenter tap, said center tap connected to said return line; a full waverectifier having a first output connected to said ringing voltage outputand having a second output connected to said negative voltage output;said full wave rectifier further comprising a first and a second inputfrom a secondary winding of said transformer T1; an over voltage protectcircuit U6 connected between said ringing voltage output and saidnegative voltage output functioning to protect said ringing voltagegenerator from a failure mode; an output switch S1 connected betweensaid ringing voltage output and said negative voltage output; a switchS2 connected to a first input of said center tapped primary oftransformer T1, said switch S2 controlled by said first driver output ofsaid push-pull switching voltage regulator circuit U2; a switch S3connected to a second input of said center tapped primary of transformerT1, said switch S3 controlled by said second driver output of saidpush-pull switching voltage regulator circuit U2; a current sensor U8connected in series with switches S2 and S3 and then connected to saidnegative output; a pulse integrator U4 driven by an output of currentsensor U8 and in turn driving said overvoltage shutdown input of saidpush-pull switching voltage regulator circuit U2, functioning to protectthe ringing voltage generator sand (SLICs) in the event of a failuremode; a differential shaper circuit U5 connected in series with ringingvoltage output and the regulation input of the push-pull switchingvoltage regulator circuit U2, providing feedback to control theswitching voltage regulator U2, shaping the ringing voltage output, andlimiting the maximum ringing voltage output to peak above said negativevoltage output; and a current limiter U3 connected in series betweensaid negative voltage input and said negative voltage output,functioning to limit the current to said ringing voltage generator if apredetermined current is exceeded; and said ringing voltage pulse ratedriving said ON/OFF input to said push-pull switching voltage regulatorU2 and driving a control input of said output switch S1 functioning toturn on said push-pull voltage regulator U2 when the ringing pulse ispresent and functioning to short any capacitance on said ringing voltageoutput with output switch S1 when said ringing pulse is not present. 2.The ringing voltage generator of claim 1, wherein said first frequencycontrol element is a crystal.
 3. The ringing voltage generator of claim1, wherein said first frequency control element is a ceramic resonator.